Hong Kong police officer Kit (Wu Jing) goes undercover in order to catch Mr. Hung (Louis Koo), the mastermind behind a crime syndicate. When the operation goes sour, and the undercover cops are betrayed, Kit disappears without a trace. Uncle Wah (Simon Yam) defies the order from his commanding officer and tracks Kit to a prison in Thailand. Thai Police officer Chai (Tony Jaa) becomes a prison guard in order to raise money for his daughter who has leukemia. He’s assigned to keep an eye on Kit. Even though Chai and Kit are in opposing positions and they don’t speak a common language, Kit turns out to be a suitable bone marrow donor who can save Chai’s daughter.